Appearance of real world surfaces is rather complicated, as it varies with the type and direction of illumination as well as with the direction from which it is viewed. Because of this complex variation, measurements of the bidirectional reflectance distribution function (BRDF) are important. The BRDF represents the amount of light reflected from a surface point as a function of four variables (polar and azimuth angles of illumination as well as the polar and azimuth angles of the viewing direction). For many applications, the quantity of interest is light reflection from the entire surface, not just a single surface point. As such, spatially varying BRDF measurements are important as well. For compactness, we refer to a spatially varying BRDF as the bidirectional texture function (BTF).
Capturing surface appearance is important for a large number of applications and industries. In general, any application that uses a computer vision system to recognize and/or classify a surface needs a BRDF/BTF measurement device to support algorithm design, development and testing. Any application that uses computer graphics rendering to synthesize an object's appearance (e.g., on a computer screen) needs a BRDF/BTF measurement device to verify how appearance should be rendered.
In the area of dermatology, a BRDF/BTF measurement device has significant potential. In clinical settings, a BRDF/BTF measurement device can enable remote and/or computer-assisted diagnosis of skin disorders. Quantitative evaluation of the effectiveness of treatments for skin disorders can also be realized with this type of measurement device. In addition, the device can provide quantitative skin appearance assessment to aid in the design, evaluation and marketing of cosmetics.
In industrial settings, measurements of the BRDF/BTF of textiles and coatings can be used for quality control and inventory characterization. Also, measurements of BRDF/BTF of materials can be used in design and planning by enabling accurate rendering prior to manufacturing. Interior design applications can use BRDF/BTF measurements for visualization purposes. This visualization is especially useful for e-commerce solutions that require the consumer to view the appearance of fabrics, wallpaper, wall coatings, etc. under a variety of orientations and illuminations. BRDF/BTF measurements enable a more complete digital representation of any product and therefore are useful in many areas of marketing and advertising. Other applications utilizing BRDF/BTF measurements include watermarking of items for preservation and security and camouflage for the defense industry.
Measurements of the BRDF/BTF are not simple. Most existing technology falls short of the ideal features for such a device. For practicality, the device should be fast and convenient. The apparatus should have as few parts as possible to minimize cost and maximize reliability. The number of moving parts should be minimal, and moving parts should take simple paths.
Measurements of the BRDF are often made using a gonioreflectometer where the illumination and viewing direction are varied over the hemisphere using mechanical means. This brute force method of moving a camera and a light source in all possible 3D directions results in a complex and expensive mechanical device. Such conventional, mechanical devices are difficult to make portable. A portable apparatus is important, because in-field measurements are desirable or necessary for many applications.
Some BRDF measurement methods avoid using a gonioreflectometer by imaging an object that has uniform reflectance and global surface shape so that a varying surface normal of the object leads to multiple reflectance measurements. However the assumption of uniform reflectance is quite restricting and limits the utility of this approach.
Also, measurement of the BRDF at a surface point is not the only item of interest. Instead, devices must have the capability of measuring the BRDF over an extended sample to capture the spatially varying BRDF (BTF). Existing devices for BRDF measurement do not enable convenient, precise control over illumination over a wide range of angles with minimal device components. Another consideration is that the device must be structured so that an extended sample is not re-illuminated by light reflecting from device components. Furthermore, the device must be capable of measuring extended samples that are not globally planar (e.g., measuring the spatially varying BRDF of skin on a human face).